# CVE-2026-31709: smb: client: validate the whole DACL before rewriting it in cifsacl

> A malicious SMB server can trigger out-of-bounds memory access in the Linux kernel SMB client by sending a malformed DACL, potentially leading to code execution or crashes.

- **Severity:** High 8.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 1 May 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-85386, BELL-CVE-2026-31709, DEBIAN-CVE-2026-31709, GHSA-8hp2-qmc6-f97h, ROOT-OS-DEBIAN-12-CVE-2026-31709, ROOT-OS-DEBIAN-13-CVE-2026-31709, ROOT-OS-UBUNTU-2204-CVE-2026-31709, ROOT-OS-UBUNTU-2404-CVE-2026-31709, UBUNTU-CVE-2026-31709

## What is CVE-2026-31709?

CVE-2026-31709 is a high-severity memory corruption vulnerability in Linux kernel, published on 1 May 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

smb: client: validate the whole DACL before rewriting it in cifsacl

build_sec_desc() and id_mode_to_cifs_acl() derive a DACL pointer from a
server-supplied dacloffset and then use the incoming ACL to rebuild the
chmod/chown security descriptor.

The original fix only checked that the struct smb_acl header fits before
reading dacl_ptr->size or dacl_ptr->num_aces.  That avoids the immediate
header-field OOB read, but the rewrite helpers still walk ACEs based on
pdacl->num_aces with no structural validation of the incoming DACL body.

A malicious server can return a truncated DACL that still contains a
header, claims one or more ACEs, and then drive
replace_sids_and_copy_aces() or set_chmod_dacl() past the validated
extent while they compare or copy attacker-controlled ACEs.

Factor the DACL structural checks into validate_dacl(), extend them to
validate each ACE against the DACL bounds, and use the shared validator
before the chmod/chown rebuild paths.  parse_dacl() reuses the same
validator so the read-side parser and write-side rewrite paths agree on
what constitutes a well-formed incoming DACL.

Weakness: CWE-1288 (Improper Validation of Consistency within Input), CWE-125 (Out-of-bounds Read)

## Which versions are affected by CVE-2026-31709?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-31709?

Apply the vendor's patch.

## Is CVE-2026-31709 being exploited?

There is no known exploitation of CVE-2026-31709 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-31709 and fix it if it is affected.

What it is: CVE-2026-31709 is a high-severity memory corruption vulnerability in Linux kernel, published on 1 May 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-31709 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-31709/ (Markdown: https://insights.sbomify.com/CVE-2026-31709/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-31709
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31709
- OSV: https://osv.dev/vulnerability/CVE-2026-31709

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-31709/
Summaries are generated and may contain mistakes; the official description above is the record.